1. Field of the Invention This invention relates to alignment systems for lithography and more particularly to darkfield alignment systems.
2. Description of Related Art
Reverse darkfield (RDF) alignment systems measure the total light scattered by an object that is scanned beneath a projected spot of light. The total energy contained in the image of the spot then forms the alignment signal, which is measured as a function of position during the scan. The optics are configured in such a way as to exclude specularly reflected light from the target. This is often accomplished with an annular mirror that causes the image at the detector to be formed only from light that is scattered by features within the illuminated volume. Light reflected from featureless flat regions of the substrate is not collected by the mirror.
These systems are considerably less sensitive than bright field systems to extraneous topography that may be present in the vicinity of alignment mark edges, such as the modulation induced by the mark in the overcoating resist layer, or topography from previous process levels. Scanned systems permit small-field imaging as well, which provides relief from off axis lens aberrations and the associated errors in alignment mark position. Small-field imaging also makes scanned systems less vulnerable to stray-light than systems using full-field illumination. A distinction is sometimes made between stray-light scattered by extraneous features at a large distance (vertically and horizontally) from objects of interest, and the extraneous signals scattered from features in very close proximity to the mark edge.
Even in state-of-the-art RDF (reverse darkfield) systems, both kinds of scattered light contribute to an erroneous background signal. This background signal will result in alignment offsets, if not adequately discriminated from alignment mark signals. Under most conditions the mark edge will produce a reasonably clear pulse of scattered energy at the detector as the edge scans through the central peak of the scanned spot. However, this pulse must be registered to within a very small fraction of its width in order to meet alignment tolerances for sub-0.5 .mu.m lithography. Thus, relatively weak extraneous signal modulation from more dimly illuminated scattering features can still produce an unacceptably large offset in the perceived mark position. A clear indication of this vulnerability can be found in the common observation that the measured edge-response of lithographic alignment systems is very much broadened when the test object is a resist coated step. Under unfavorable conditions, the accumulated scattering from grainy films and the like can completely obscure the signal from the mark edge.
X-ray lithography alignment systems have a particular vulnerability in this regard, due to the the close proximity of the mask and the wafer. In such cases, the illuminated region must be regarded as a three-dimensional cone having substantial volume, even though the cross-sectional area in the image plane is quite narrow. For example, when an x-ray alignment system views the wafer marks through mask windows, scattered and specular noise from the mask corrupts the information in the wafer alignment signal, leading to significant background noise (FIG. 1). Currently available commercial alignment systems suffer from a related sensitivity in that the wafer alignment grating is viewed directly through the mask alignment grating; and both gratings scatter light into the image plane simultaneously. FIG. 1 shows the manifestation of optical stray light in a prior art system with a wafer 10, and a mask 11 with an aperture 12 therethrough. An illumination ray 14 is directed through aperture 12 to hit wafer 10. The ray 14 is reflected up as beam 14'. Specular noise "beams" 15 are reflected from the surface of the mask at the aperture 12, backscattered noise 15 is also reflected from the surface of the mask 11 at the aperture, and forward scattered noise 17 passes to the wafer 10 where it is reflected as scattered noise 18 which passes back through the aperture 12 above mask 11. The beam 14', as well as the specular noise 15 and scattered noise 18 all pass toward collection area 19 above mask 11. The mask specular noise "beams" 15 and scattered noise 18 corrupt the aligner signal. As the gap is lower, (40 .mu.m.fwdarw.20 .mu.m), the problem of scatter and corruption of the alignment signal increases.
U.S. Pat. No. 4,672,557 of Tamura et al is not confocal and does not use darkfield illumination. The CCD processing used does not use the same principle. A different signal processor is described.
U.S. Pat. No. 4,814,829 of Kasugi et al does not use confocal spatial filtering and it uses a standard darkfield system. Filtering is in the pupil plane, not the image plane. Furthermore, the shape of the filter is not matched to the instantaneous image of the alignment mark.
U.S. Pat. No. 4,663,534 of Ayata et al is not confocal, i.e. the filtering aperture is not sub-resolution to the imaging system. This embodiment is similar to '829.
U.S. Pat. No. 4,614,431 of Komeyama relates to a dual focus alignment system for proximity alignment.
U.S. Pat. No. 4,744,666 of Oshida et al is not confocal and "out of conjugate relation" (Col. 1, line 60)
U.S. Pat. No. 4,947,413 of Jewell et al filters in the diffraction plane. Spatial filtering is done in the frequency domain and not in image space as our invention proposes.
In accordance with this invention, a microlithography alignment system:
a. employs a reverse darkfield configuration, and PA1 b. includes means for confocal spatial filtering, whereby the optical signal is improved and alignment offsets are reduced, providing a confocal darkfield alignment system.
Preferably the means for confocal spatial filtering employs a two dimensional CCD detector array or diode array detectors as a confocal filter. In one aspect of the invention the means for confocal filtering employs empirical optimization of the filter by reweighting the CCD array and correlating back to measured overlay results. Alternately, the filter pixel weighting can be optimized based on an alignment signal Figure of Merit. This Figure of Merit can be based on either the alignment signal symmetry and peak amplitude or based on information known a priori in the alignment target design.
An example of a Figure of Merit based on a prior information can be found in "Use of A Priori Information in Centerline Estimation" by A. Starikov, A. D. Lopata and C. H. Progler, Proceeding of the Microlithography Seminar, Interface '91, 277-285 (1991) Here, alignment signal symmetry, redundancy and statistics are used to assess the validity and quality of acquired alignment data. The confocal darkfield pixel weighting can be done in real time to improve these attributes of the alignment data.
Many standard estimates of signal symmetry such as comparing opposing edge slopes or scattered amplitude of opposing edges can also be used as a Figure of Merit during window optimization.
In another aspect of the invention, the confocal spatial filtering employs a quadrant detector as a confocal filter.
In the configuration where the filter is matched to the instantaneous darkfield image of a single alignment mark edge, the means for confocal spatial filtering is comprised of a filter consisting of a double slit.
Instead of matching the pixel weighting to the confocal darkfield instantaneous image of a single isolated edge (i.e. a double slit filter) the filter can be matched to the instantaneous image of a double bar target. The two bars should be unresolvable to the projection NA (Numerical Aperture) but clearly resolved by the collection NA. This pattern can be collected many times during a scan improving the signal-to-noise ratio.
The spatial filter is of the electronic variety such as a CCD array, or mechanical variety such as sliding shutters.
The alignment system configuration is used for dimensional metrology measurement of integrated circuit patterns.
The alignment system is used to provide automatic focussing feedback to position stages for a positioning application.
The collection and illumination apertures to form confocal images for detector array filtering can be adapted in real time to optimize measured overlay results or the Figure of Merit.
In a preferred aspect of the invention, a system with one or more axes of physical scan motion are replaced by a virtual scan operation. The virtual scan motion resulting from orienting the projected reference slit image at an angle to the CCD array axis and the alignment mark being viewed, by electronically projecting the CCD array pixels along the axis for the alignment mark under inspection, the centerline of said alignment mark being localized with no physical motion of detector or target.